Treatment of rubber articles



Patented Dec. 14, 1937 TREATMENT OF RUBBER ARTICLES Abraham K. Brill and Walter A. Wake, Young'stown, Ohio, assignors to The Republic Rubber Company, Youngstown, Ohio, a corporation of Ohio No Drawing.

Application August 17, 1935,

Serial No. 36,753 r a k '7 Claims. (Cl. 260-1) This invention relates to the treatment of soft cured rubber and more particularly to a process for so conditioning the surface of the rubber as to generally improve the characteristics thereof.

tion that there is eliminated the danger of the pulley "seizing the rubber belt and so causing the rubber to melt through to result in a broken belt. Of course, many other examples of rubber it is advantageous to provide understood therefore that it is within the contemplation of the present invention to provide gens we have found. that bromine and chlorine may be most advantageously employed, while of the mineral acid group concentrated sulphuric and nitric acids are the most desirable.

a hard, tough, nontacky and highly polishable surface and it is to be 5 It is well-known in the art to which this invenvarious and sundry rubber products with surfaces 5 tion pertains that when soft rubber is vulcanized having the characteristics and attributes hereinby any of the usual methods, even when cured before mentioned, the invention being not limited under the best of conditions, as by the use of to any particular product but rather to theprochighly polished molds, the surface of the rubber ess for surface-conditioning the rubber of which product has a characteristic non-slippery finish the product is formed. 10 with a high coeflicient of friction. Such surfaces We have found that the desired surface charare easily marked, quickly become dulled and acteristics are obtainable by subjecting the rubber roughened in handling, and are readily suscepproducts to certain chemicals which may be tible to becoming dirty and stained, in consebroadly classified in two groups as follows: the

quence of which the product soon acquires an unhalogens and the strong mineral acids. In the 15 sightly appearance. Attempts have been made halogen group are included the following elefrom time to time to improve this condition by the ments:bromine, chlorine, fluorine and iodine; application of various finishes, such as flexible while in the mineral acid group the following varnishes, lacquers, paints and other such surface materials are inc1uded:nitric acid, sulphuric coatings but all of the latter have generally proved acid, hydrochloric acid, chromic acid, hydriodic 2 to be unsatisfactory because of their uneven acid and hydrobromic acid. We have found that wearingv qualities, their tendency to peel and chip when any of the above chemicals are employed away from the rubber surface, and their generally in treating the surface of a soft cured rubber unsightly appearance after the product has been article it reacts with the surface of the rubber in use for only a relatively short time. to form, in combination with the rubber, a new 25 It is among the principal objects cf the present surface which istough, resilient and readily polinvention' to provide a treatment for soft cured ishable and which has a characteristic that can rubber of such character that there is imparted best be described as being slippery smooth." thereto a surface which is at once tough, elastic While it is known that rubber is fairly reactive and so hardened that it is not easily marked by with certain chemicals, the mechanism of the re- 30 dirt, stains and other such disfiguring substances, action is not definitely known and accordin y which is highly polishable, and which has a no attempt is made herein to express definitely non-tacky, slippery smooth feelnot characteristic what reaction, if any, is obtained when the rubber of ordinary soft rubber. is subjected to the treatment of our present in- A furtherobJect is to produce soft cured rubber vention, Rather, we merely venture the observa- 35 l articles the surfaces of which are highly lesifiiant tion that the combination formed upon the surto 01 sun ht and oth r su h ments whi face of the treated rubber may possibly'be likened ordinarily tend to induce surface cracks and t t t bt i by ordinary sulphur m m checks in articles formed of rubber and the like. tion, the difference being that the product ob- 4 AS has been indicated above, rubber articles tained when subject to our present treatment 40 which have been subjected to the surface-condiis provided with a surface which is harder, tougher tionin treatment of he present invention are and more readily polishable than that obtained by materia y i p ed not only in their al apfurther vulcanization of the rubber with sulphur pearance, due to the high polishability of the monov treated surface, but also in their physical characwmle any of the above halogens or mineral 45 teristics. Fo a e. in the case o a -rubb acids may be employed, either singly or in compulle'y belts, when the latter are surface-finished t1 to produce the desired surface c nd in accord with the inventionto be tly tion, we have found that certain of them are described. the y. smooth surface promore preferable to use due tothe greater facility duced has such a relatively low coefficient of frict which they may be handled. Of the haloo products may be cited in connection with which In the use of the halogens, the treatment may 55 invoive'subjecting the rubber products to one or another of the halogens or to a combination thereof with the halogen or halogens in a purely gaseous state or in a liquid solution. I'hus, the rubber products may be placed in a chamber filled with bromine vapor or chlorine gas or a combination thereof, or the rubber products may be immersed'ina saturated aqueous solutionxof of these acids.

bromine or chlorine or' a combination thereof. Where it is desired to employ iodine as the surface conditioning agent the iodine may be dissolved in alcohol or ether or in an. aqueous solution of potassium iodide, hydrogen iodide or anyother iodide, the rubber products to betreate ed being immersed in the'iodine solution so provided.

Fluorine, while not so easily handled as the other halogens, may be employed in a gaseousproducts to be treated may be immersed.

In the useL-of the mineralacids above referred to, the rubber products to be treated are preferably immersed in allquid bath of one or another In this connection we have found that a mixture of concentrated nitric acid and concentrated sulphuric acid produces most desirable results and we prefer such mixture of these acids to the use of either acid alone because of the greater ease of handling of the mixture and the more uniform surface-conditioning results obtained. Certain of the mineral acids may be employed, if desired, in their gaseous state, as, for example, hydrochloric'and hydrofluoric acids, these acid gases or vapors being placed in sealed chambers within which the rubber products to be treated are also placed.

It is preferable, upon removal of 'the rubber products from the treatment medium, partie --v larly where it consists of non-volatile acids, such as nitric and sulphuric acids, to wash any excess of the reacting agent from the surface of the rubber product. To this end, immediately upon removal of the rubber products from the treating medium, they are washed with water, following which the treated surface may be' further washed with an alkali, such as dilute ammonia,

to neutralize any traces of the reacting agent which may have adhered to the surface of the rubber. as a neutralization agent because it is completely volatile and leaves no residue.

In the use of these surface-conditioning agents, it is not possible to state definitely the exact time which must elapse while the rubber product is being subjected to the influence of a particular surface-conditioning agent. The period during which the rubber product is immersed withinthe liquid treatment bath or subjected to the gaseous treatment depends, of course, not only on the particular chemical which is employed and its form, but also on other factors, such as'concentration of the 'surface-cond.itioning medium, its temperature, and the type of surface which it is desired to produce upon the rubber product being treated. Preferably, the treatment is performed at room temperature, any substantial variations in-which might require some compensatory change in the length of the period during which the articles to be treated ard subjected to the treatment medium being currently employed. Inasmuch as the rubber products may vary materially in their character and thus re- Dilute ammonia is preferably employed present treatment in order .to determine the best period of exposure for a particular article, to first expose the article to a particular reacting agent for a minimum period and then finish it immediately by polishing, this operation being repeated for gradually increasing periods of exposure for that particular article. The most effective period of exposure thus obtained after these preliminary trailsis then determined as the optimum dripping time to be used for any current "production of that particular product for a given surface-conditioning medium, which latter may 'be' either in liquid or gaseous state as describcd above. As a specific example, where a. mixture .of eighty parts of concentrated nitric acid and twenty parts of concentrated sulphuric acid was employed as the surface-conditioning medium fora lot of vacuum cleanenhandles, it was determined after several preliminary trial treatments that the best surfacing results were obtained when the rubber handles were immersed for a period approximating fifteen seconds. The period of exposure may, however, vary from five seconds to ten minutes or more, depending upon the type of treating medium employed and the type of surface desired to be obtained. For example,.where bromine solution is employed it may be necessary to expose the articles to the influence of the reacting medium for as much as ten minutes to obtain a desired surface condition, while in the case of hydrochloric gas the time of exposure may extend to as much as six to eight hours.

If the time of exposure is extended over too long a period, the hard skin which is formed upon the surface of the rubber article may be too thick with the result that the external surface thereof becomes so brittle that checks and cracks are produced on the surface. For example, if the vacuum cleaner handles above mentioned had been dipped for say three minutes instead of for fifteen seconds, their surface would have been so deeply cracked that it could not have been finished in the desired manner. Accordingly, once an optimum dipping time is established for a given rubber article to be treated and for a given treatment medium, that time must be adhered to within reasonable limits if uniform results are to be obtained for any current run of a lot of such articles.

Following the step of subjecting the rubber articles to the surface-conditioning treatment as hereinbefore described, the articles so treated, may be dried and are then finished by polishing in any desired manner, as, for instance, by tumbling or by the application of a polishing wheel to the treated surface. With the surface preliminarily conditioned in the manner herein before described any desired degree of lustre may be given to the surface of the rubber article.

What is 'claimedas new and useful is:

1. A method for surface-conditioning an article formed at least partially of rubber which consists in subjecting the rubber surface of the article to the action of a solution containing nitric acid for a length of time sufllcient to form a hard polishable skin thereon.

2. A method for treating an article having a rubber surface which consists in initially immersing said surface in a nitric acid bath; in maintaining said surface immersed for a period of time suilicient to form upon said surface a hard,

polishabie skin; and in washing the treated surface free of said nitric acid upon removal from. the bath.

3. A method for treating an rubber surface which consists jecting said surface to the action of nitric acid; in maintaining said surface immersed for a period of time sufficient to form upon said surface a hard, polishable skin; and in neutralizing any excess acid which may adhere to said surface upon removal thereof from the bath.

4. A soft, cured rubber article having a ground, highly polished nitric acid hardened surface layer. I

5. A soft, cured rubber article having a ground, highly polished hardened surface layer formed in article having a in initially subsitu by a, surface-conditioning agent containing essentially nitric acid.

6. A method for treating an article having a rubber surface which comprises subjecting the surface thereof to the action of a surface conditioning agent containing essentially nitric acid until a hard, highly polishable surface is formed,

and grinding the treated surface to polish the same.

7. A method for surface-conditioning an article having a surface of rubber, which comprises immersing the article in a treatment bath containing in solution a mixture of nitric and sulphuric acids in the ratio of 86 parts of nitric acid to 20 parts of sulphuric acid.

ABRAHAM K. BRILL. WALTER A. WAKE. 

